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the class of enzymes that catalyze oxidation-reduction reactions. Oxidoreductases are present in all living cells. The oxidized substrates that oxidoreductases act on can be a hydroxyl group (—OH), aldehyde group (—CHO), ketone group (> CO), or ethyl group (—CH2—CH2—). This class of enzymes also acts on reduced forms of pyridine coenzymes, for example, nicotinamide-adenine dinucleotide (NAD) and nicotinamide-adenine dinucleotide phosphate (NADP). Among the substances that are reduced in an oxidoreductase reaction, that is, among those that serve as acceptors of hydrogen and electrons, are NAD, NADP, cytochromes, lipoic acid, and quinones. The most important oxidoreductases include dehydrogenases, which transfer hydrogen and electrons during respiration and photosynthesis; oxidases, with the oxidizing agent O2; peroxidases, with the oxidizing agent H2O2; hydroxylases, which incorporate one atom of O2 into a molecule of substrate; and oxygenases, which incorporate two atoms of O2 into a molecule of substrate. More than 200 oxidoreductases are known.